Experimental study of radial electric field and electrostatic potential fluctuation in the Large Helical Device

A heavy ion beam probe was installed on the Large Helical Device (LHD) to investigate the roles of radial electric fields (Er) in magnetically confined high-temperature plasmas. Two new observations are presented. One is the observation of electrostatic potential profiles during the formation of extremely hollow density profiles of impurities, called the impurity hole (Ida K et al 2009 Phys. Plasmas 16 056111), in the LHD plasmas. The measured Er is negative, and the Er determined by the ambipolarity condition of neoclassical particle fluxes is consistent with this observation. However, the transport analysis indicates that the formation of the extremely hollow profile is not attributable to the impurity fluxes driven by Er and the density and temperature gradients of the impurity. The other new observation is on the geodesic acoustic mode (GAM). The electrostatic potential fluctuation associated with the GAM, which is probably induced by energetic particles, in plasmas with the reversed or weak magnetic shear is identified. The GAM is localized in the core region of the plasma.